Conventional touch panels and touch screens utilize electrical resistance, electrical capacitance, infra-red light, or surface acoustic waves to determine when an object has contacted the surface of the touch panel. Acoustic wave touch panels use principally out of plane acoustic wave motion to detect contact between the touch panel and an object. However, out of plane acoustic waves are sensitive to liquid and sealing compounds on the touch surface.
Acoustic wave touch panels utilizing in plane acoustic waves have been developed to address the problems of out of plane acoustic touch panels. However, these panels are limited to applications where the frequency thickness product of the touch panel is less than 2.5 MHz-mm for single mode operation, which necessitates a substrate thickness of 1 mm or less. Consequently, in plane acoustic wave touch panels generally require a laminated backing substrate for strength. In addition, a wave generator is restricted to being positioned on an edge of the substrate, or sensing film, of the in plane acoustic touch panels, for proper operation. As a result, in plane acoustic touch panels have not enjoyed commercial success.
To overcome these issues, some acoustic touch panels incorporate acoustic in plane motion in a series of acoustic switches to determine regions where an object comes into contact with the touch panel. These touch panels include a plurality of acoustic touch switches that are arranged across the surface of the touch panel. Each acoustic switch is coupled to a respective in plane overtone mode wave generator, which transmits a wave that is trapped in a cavity in each acoustic switch. As an object makes contact with one or more of the acoustic switches, the corresponding wave generators for those switches determine the region where the object has contacted the touch panel. Because each switch is coupled to a wave generator, a touch panel utilizing acoustic touch switches can be manufactured with the wave generator positioned on the top surface of the touch panel. Further, the acoustic touch switches can be incorporated into panels having a frequency thickness product much larger than 2.5 MHz-mm. However, because each acoustic touch switch requires a wave generator, the cost and complexities of these touch panels is cost prohibitive.
Accordingly, a need exists for an acoustic touch panel having a reduced number of transducers that operates on a touch panel having a frequency to thickness product larger than 2.5 MHz-mm.